1. Field of the Invention
This invention concerns improvements in compaction formed agricultural granules containing calcium sulfate (gypsum) adapted for use on soil to provide nutrients to plants and as a carrier for fertilizers, pesticides and the like. More particularly, this invention is concerned with improving such compositions by providing them with controlled availability to plants of sulfate nutrient and by providing them in a form that is hard and scarcely dusting in a sulfate-fortified agricultural granular product. Further this invention relates to novel compositions; and more particularly to novel compositions which supply sulfate as a plant nutrient and soil amendment in a controlled, time-release fashion.
As is well known in the art, pelletilization is an agglomeration process in which powdered and liquid materials are rolled around a disk or drum surface without pressure and allowed to agglomerate by mere physical attraction and gravitational forces. In contrast thereto, pressure roll compaction as set forth in U.S. Pat. No. 3,820,970 to Watkins is an agglomeration process wherein powdery material is compacted under pressure into a sheet which is then flaked into chunks and then crushed and screened into desired size of particles. While in the agricultural industry drum and disk pelletizers and pelletizing methods are sometimes referred to as granulators and granulation; herein the terms pellets, pelletizing and pelletizor will be used to describe non-pressure agglomeration while the phrase granule and granulator will be used with reference to pressure compaction agglomeration techniques and products.
Sulfur is one of the many elements essential for plant growth. It is a macronutrient, like nitrogen, phosphorus, potassium, calcium and magnesium; and must be available in relatively large amounts for good crop growth. Sulfur is also useful as a fungicide and insecticide. At one time sulfur was a common component of many low analysis fertilizers. Such fertilizers satisfied most needs for sulfur fertilization of agricultural crops. If and when additional sulfur was needed, powdered gypsum (land plaster or calcium sulfate dihydrate) was commonly used. However the development of newer, high analysis for the other macronutrient, fertilizers has resulted in the more frequent and extensive reporting of sulfur deficiency in soils throughout the world. In addition, increased crop yields, double cropping in many areas, and the greater emphasis on control of air pollution and consequent utilization of low sulfur fuels thus reducing atmospheric sources of plant nutrient sulfur have aided and abetted increasing sulfur deficiencies. Thus it has become important to provide sulfur application to soils.
Elemental sulfur itself is, of course, the most concentrated sulfur carrier for treatment of soils. However elemental sulfur is a form not readily and immediately utilizable by plants. Generally, before most plants can utilize elemental sulfur it must first be converted to a sulfate form (SO.sub.4) in the presence of soil microorganisms. In many cases, thus, application of the elemental sulfur must be made more than one growing season ahead of the time when it is needed and will be available for utilization by the plants. Since sulfur deficiency is steadily increasing particularly in areas of double cropping and in connection with increased crop yields, this at times is of benefit rather than disadvantage. However, from the farmer's point of view, the most economic and convenient method of applying needed plant nutrient sulfur is likely to be in connection with his fertilizer treatment for a current growing season.
2. Description of the Prior Art
The most straightforward manner for applying the most concentrated sulfur to soils is by the application of elemental sulfur. Since elemental sulfur is generally applied as a dust, it is subject to being blown away in the winds or washed away by flowing surface water or soak into the ground below the plant roots. In these cases it never becomes available to the plants. Further, since it must convert to the sulfate form before becoming available to the plants for utilization, application of elemental sulfur powder may provide sulfate availability for plant utilization at an unpredictable time after application to the soil. This frequently results in the necessity to refertilize the plants in order to maintain plant growth.
One method of reducing the time delay after application of sulfur before it can be utilized by the growing plants is to provide it in a sulfate form. Thus it has been a long standing practice on certain soils for certain plants to apply finely particulate gypsum; which treatment is commonly referred to by those in the industry as "land plastering". As a powder form of application, there may be mentioned U.S. Pat. No. 3,846,116 which teaches the application of calcium-containing compounds, such as calcium sulfate, in finely divided powder form, more particularly of having a size of about 20 microns or less and preferably of sub-micron size in the treatment of peanut plant foilage to prevent the formation of "pops" and unsound kernals. That patent mentioned that as a physical admixture of powders there might also be included sulfur along with the calcium-containing compound in a dust or powder of the requisite particle size. This is a physical admixture in the form of a dust or powder of the requisite small particle size. Again, since the form being applied is a powder it is subject to erosive forces, as well as the inconvenience of enveloping the applicator in a "cloud of dust." Further, such treatment must be at the appropriate time coincident with the individual plant's needs in the growing seasons. In addition powder forms are difficult to handle by the farmer's typical fertilizing equipment.
There have been a number of attempts to incorporate elemental sulfur into more intimate admixture of macronutrients to be provided in fertilizer-sized agglomerated form. Principally these have been concerned with forming a solvent solution of the elemental sulfur, along with other substrates, and using these as coating around the fertilizer nutrients. Such practices are exemplified by U.S. Pat. Nos. 3,295,950; 3,342,577; 3,576,613; 3,580,715. In addition to these, U.S. Pat. No. 3,799,884 discloses a soil application sizes of elemental sulfur produced by aqueous dispersion grinding of a "porous particulate sulfur." Further, U.S. Pat. No. 2,416,663 teaches the production of a pellet fertilizer composed of a by-product of molten sulfur extraction from phosphate gange material in pelletized fertilizer form.
More recently U.S. Pat. No. 3,820,970 discloses a method of compaction granulation of calcium sulfate utilizing stucco as a binder to provide gypsum is a hard, less dusty granular form more convenient for application to the soil. It is apparent from this patent that if gypsum particles are pressure compacted without such a binder, the particles will not hold together but will disintegrate in handling causing "dusting". That is, when ground gypsum is subjected to pressure, in attempts to compact it into a sheet and then break the sheet up into granules, the particular characteristics of the calcium sulfate material are such that an inordinate amount of fine, powdering dusts and irregular-sized particles result unless suitable binders or equivalent are included.